Lithium ion batteries have replaced traditional batteries in the fields of mobile phones and portable computers etc., due to their advantages of higher working voltage, higher energy density, larger output power and longer cycle life. Lithium ion batteries with high capacity have been widely used in electric vehicles, and will become one of the main power supplies of electric vehicles in the 21st century.
In order to endow the electric vehicles with longer mileages, higher requirements to the energy density of batteries have been put forth. Conventional anode active materials of commercialized lithium ion batteries are mainly carbon materials, such as graphite with a theoretical capacity 372 mAh/g only. This has greatly limited the improvement of the energy density. Because of this, new generation of anode active materials is under development, which has a theoretical capacity significantly higher than that of conventional carbon materials. For example, materials containing Si, Sn, or the like, which are capable of alloying lithium with ease. Since silicon (Si) has a theoretical capacity (4199 mAh/g) much higher than that of conventional carbon materials, which offers a prospect of greatly increasing the battery capacity, it has become research hotspots in anode field recently.
Theoretically, the battery capacity will be improved when using silicon or the like with higher theoretical capacity as the anode active material. However, silicon has disadvantages as below: it will strongly expand or contract during the charge/discharge process, which leads to a large volume change greater than 300%. This tends to cause cracks or even pulverization near its surface. Further, cracks on anode materials will reduce the electrical conductivity of the materials, and create new active surfaces. The new active surfaces can accelerate the decomposition of the electrolytes and form a film on the new active surfaces. These will badly affect the cycling performance of the lithium ion batteries. In addition, conventional anode plate is prepared by the following steps: first, mixing an anode active material, a binder, a conductive agent and a solvent etc. to form slurry; and then coating the slurry onto an anode current collector. During the charging or discharging, violent volume change will weaken the adhesion between the active material layer and the current collector. Even if fluorine-containing resin or the like is used as the binder, it is still difficult to maintain good contact between the active material layer and the current collector. Along this, the active material layer will inevitably peel off the current collector, and the cycling performance of the lithium ion batteries will rapidly decrease.
To solve the problems above and restrain the silicon expansion, conventional methods include the following: limiting the size of silicon particles (for example, making silicon nanoparticles, silicon nanowires or silicon nanotubes), compositing or coating with carbon materials. However, these methods provide limited benefits in improving the performance of silicon anode materials. Nano silicon particles are very prone to accumulate, which makes the nano materials cannot maximize their advantages; meanwhile, as regard to compositing or coating with carbon materials, the bonding between the silicon and carbon elements is weak, which makes the silicon anode material cannot deliver its electrochemical performances. Moreover, the above preparation methods are complicated, which will obviously add to the cost of lithium-ion battery.